As Franchesoa explains the problem seems to arise when there are strong contrasting colors in the grid. In your case: Dark images and white background.

This page describes how the problem can be dealt with when styling tables:

"Avoid using dark and heavy grids".

Another post I found discussing this (and other) optical illusion(s) states the same answer:

The Hermann grid illusion will not work if there is a low contrast between the colours involved in the grid. As a sidenote, this is why Excel has light blue cells with a white background: there is not enough contrast between the light blue and the white colours to confuse the visual system, so the illusion does not appear.

Try considering a different color for the background, like a soft gray or blue to ease the contrast. Easing the color on the images will also help you if that is an acceptable solution.

Hope this helps

As Franchesca explains the problem seems to arise when there are strong contrasting colors in the grid. In your case: Dark images and white background.

This page describes how the problem can be dealt with when styling tables:

"Avoid using dark and heavy grids".

Another post I found discussing this (and other) optical illusion(s) states the same answer:

The Hermann grid illusion will not work if there is a low contrast between the colours involved in the grid. As a sidenote, this is why Excel has light blue cells with a white background: there is not enough contrast between the light blue and the white colours to confuse the visual system, so the illusion does not appear.

Try considering a different color for the background, like a soft gray or blue to ease the contrast. Easing the color on the images will also help you if that is an acceptable solution.